Electric amplifier



Sept. 26, 1933. K. RATH ELECTRIC AMPLIFIER Filed Jan. 23, 1931 IHVENTOI? Karl Raih BY 5. mi

HTTOENEY Patented Sept. 26, 1933 I UNITED STATES PATENT OFFICE ELECTRIC AMPLIFIER Karl Rath, New York, N. Y., assignor to Radio Patents Corporation, New York, N. Y., a corporation of New York Application .lanuary 23, 1931. Serial No. 510,599 12 Claims. (01. 179-171,-

My invention relates to electric amplifiers utilcreased operating stability and dependability of izing discharge devices and more particularly to service. direct coupled vacuum tube amplifiers. Ampli- A more specific object of my invention is the fiers of this type, in which a direct coupling is provision of a Wheatstone bridge coupling arused for handing onthe output energy of one rangement for handing on the output energy of 50 amplifying unit or valve to the input circuit of a one amplifying valve in a cascade system to the succeeding amplifying unit or valve,.have lately input circuit of a succeeding valve, which conbecome known in the art and were primarily sists substantially of ohnfic resistors and in designed for the amplification of direct or uniwhich the second valve is controlled by unbalanclateral current variations, in which case the ing 0f t e'brid e System, in accordance with Vari- 65 use of capacitative or inductive elements in the ations of input currents applied to the first valve. coupling path from one amplifying valve to the This and further objects and features of my next amplifying valve becomes prohibitive, as invention will become more apparent by the folthese devices would block the direct current comlowing detailed description, taken with reference ponent of a unilateral current variation and to the accompanying drawing, illustrating some 70 transmit only the variation itself, superimposed practical embodiments of the novel inventive on the direct component. idea. I wish it to be understood, however, that Direct coupled or direct current amplifiers of the description and the exemplifications as given this type, on the other hand, also present great by the drawing should be illustrative only of advantages when used for the amplification of the broader aspects and principle underlying the 75 pure alternating current variation, such as speech invention, as set forth in its broad scope in the or musical currents operating a loud speaker, or appended claims. the like. This advantage consists primarily in a In Figure 1 I have shown, for the sake of exconsiderablydecreased tendency to produce displanation and clearer understanding, a direct tortion of the signal current shape in comparicoupled amplifying circuit of known design,

son with the well-known amplifiers as used in known in the art. the art for this purpose, utilizing devices liable Figure 2 shows a circuit arrangement of Figto produce distortion of the current wave shape, ure 1 in an analyzed form, chosen for the sake such as capacity or inductance elements in the of clearer understanding.

coupling path from one amplifying unit or valve Figures 3 and 4 represent respectively a workto the next amplifying unit in a cascade ampliing diagram and an analyzed circuit diagram of fication system. one form of embodiment of my invention.

There exist, however, a number of inherent Figure 5isatheoretical diagram explaining the disadvantages of direct coupled amplifiers of the connection of the valves, according to Figures 3 a type referred to known in the art, which have and 4.

hithereto made their application in practice a Figure 6 is a similar circuit diagram of an very limited one. A main disadvantage of direct amplifier as shown in Figure 3, with three amcoupled amplifiers of known design consists in a plification stages, including a current supply arstrong tendency to instability, which tendency rangement from an alternating house current increases rapidly as the number of amplifying net work.

stages or units is increased. In fact, it was found Figures 7 and 8 show respectively an alternathat two or three stages were the maximum limit tive working and corresponding analyzed circuit which could be used to insure fairly eflicient and for practicing my invention.

r dependable service in operation. It is obvious, Similar reference numbers or characters idenespecially when telephone or similar complex tify respectively similar elements or entities current variations are to be translated and amplithroughout the different views of the drawing. fied, that even the slightest tendency of the Referring more particularly to Figure 1, this amplifier to become unstable renders its practical illustrates a well-known direct coupled simple r application completely prohibitive for such a amplifying circuit, comprising two amplifying 105 purpose. valves 1 and 2 of ordinary design and. construc- Accordingly, it is an object of my invention to tion, having filament, grid and plate electrodes. privide a new amplifying circuit for a direct A potentiometer 3, connected to a plate current coupled cascade amplification system, which is supply source, as indicated by the plus and minus devoid of the inherent disadvantage of such amsigns, serves for providing the operating and com 110 plifiers as above stated and which insures introl potentials of the amplifying valves. As is seen, both valves are arranged in series or staggered fashion in respect to the potentiometer, so that the resultant potentiometer voltage has to be equal to the voltage necessary for one valve multiplied by the number of valves used in cascade, which, in the present case, is two. The plate electrode of the second valve 2 is directly connected to the positive pole of the high voltage battery and the plate electrode of the first valve connected to an intermediary tap c on the potentiometer, in series with a resistor 4, commonly designated as coupling resistor. The cathode of the first valve is shown to be connected to the minus terminal a of the potentiometer. In order to prevent the high potential on the plate of the valve 1, which is directly connected to the grid of the succeeding valve 2, from afiecting the grid of the valve 2, the cathode of the latter is connected to a suitable tap point b on the potentiometer at a negative potential in respect to the pointe, so that the potential drop from c to 1) provides a suitable compensating potential difference between the grid and cathode of the valve 2, which neutralizes the effect of the high voltage plate potential of the valve 1 on the grid of valve 2. The input current variations are applied to the grid of the valve 1 at the terminals 5 and the corresponding amplified .output currents occurring in the anode circuit of the valve 2 may be utilized to operate a translating device ofany type, such as loud speaker, television receiver, etc. connected to the output terminals 6. This system corresponds to. the well-known direct coupled amplifying system known in the art up to date. As already stated, this system has a number of inherent disadvantages, which consist primarily of a strong tendency to instability during operation, on account of reaction of one valve upon the preceding valve or valves, as becomes obvious from the circuit diagram, from which it may be gathered that the anode current of one valve, by passing through the common potentiometer 3, is bound to affect the voltage or current distribution of the preceding valves, which, as a simple analysis will show, results in a kind of reaction, which has the effect of shifting the operating point; that is, primarily the grid bias point of the preceding valve or valves, to either positive or negative values, so that, with sufficient-reaction; that is, sufficiently strong plate current variations of the succeeding valve or valves, one valve in the system may become either entirely saturated and act as a current limiting device, or it may be entirely blocked through increased negative bias, thus, in both cases, acting as, a complete stop for the transmission of energy through the amplifier. As is obvious, this effect is the more imminent the higher the number of amplifying valves connected in cascade; that is, the greater the final current variations, whereby the reaction effect will cumulatively increase and cause the amplifier to completely drift to a nonoperative or unstable condition. It is for this reason that amplifiers of this type have been used for not more than 2 stages of amplification, to minimize its tendency to become unstable.

One means to overcome this disadvantage would be to use a potentiometer 3, which passes a current sufliciently high, as compared to the plate currents of the amplifying valves, so that the reaction eifect becomes negligible. For practical purposes, about 1 ampere potentiometer current will be required, considering the plate current of the. usual output or power valves, which,

' is about 30-40 milli mperes. Assuming, for instance, a three stage amplifier to require about 300 volts high potential voltage; that is, about 100 volts for each valve, the resistance of the potentiometer would have to be As the current heat dissipation in the resistance is equal to the square of the current multiplied by the resistance, the energy expenditure in the potentiometer alone would, in this example, be 300 watts, which is prohibitive in a radio receiver or the like apparatus. Therefore, it is necessary to use a potentiometer with considerably less cur-' rent, in which case the amplifier again tends to become unstable through reaction of the plate currents of succeeding valves on the operation and one leg being comprised by the input valve- 1. The operating potential is applied'to one pair of diagonal terminals (1 and c of the bridge system and the cathode and grid of the second valve is connected to the remaining pair of diagonal terminals of the bridge, as shown. The bridge in the normal state is slightly unbalanced, so as to provide the required 'negativeoperating grid bias for the second valve. When the input currentsapplied to the terminals 5 vary, this will produce a corresponding additional unbalancing of the bridge and, accordingly, a corresponding plate current variation of the second valve 2.

The operating stability of a direct coupled amplifying system is considerably increased in accordancemith the invention by using a bridge arrangement comprised substantially by resistors and by associating with it the input valves, in such a manner as to produce the desired amount of unbalancing on the system, in accordance with the input signal variations. This I have illustrated in the analytical diagram according to Figure 4, which illustrates a complete bridge system comprised of a resistor for each arm of the bridge and the'input tube 1 connected in parallel to one of the arms. The working diagram for such a circuit is shown in Figure 3, which differs essentially from Figure 1, in that at least two potentiometers 3 and 7 respectively are provided connected in parallel to the high potential supply terminals. plates and cathodes of the individual valves are then connected to suitable points respectively to each of these potentiometers, as illustrated. As-

The.

plus terminal of the high voltage battery. The

filament of the second valve is connected to a point e on the lower potentiometer 3, which is somewhat ahead of the "100 volt potential point-say at about 105 volts, so that the effective plate potential of the valve 2 is 95 volt and-its negative grid bias equal to the difference of the plate potential of valve 1 and the filament potential of valve 2; that is, equal to 100 minus 105 -5 volts, which it is assumed corresponds to an efficient and favorable operating point on the rectilinear grid voltage-plate current characteristic of the valve. By suitably adjusting the taps e or ,f on the potentiometers 3 or '7 respectively, it is obvious that any desired negative biasing voltage may be readily adjusted. The same may be done for the valve 1 by adjusting the tap points 11 and g respectively on the potentiometers 3 and '7, as shown. This isstill further explained by Figure 5, in which the two parallel horizontal lines represent the levels of potential as supplied by the high potential source and the two dotted horizontal lines .1: and y, also shown in Figure 3, represent the levels of potential as applied to the plate of valve 1 and grid of valve 2 respectively and to the filament of the valve 2. The difference in level between :0 and 3/ has to be chosen so as to present the proper operating bias for valve 2.

Referring to Figure 6, I have illustrated a complete operating circuit of a direct coupled amplifier, in accordance with the invention, which may be operated directly from an alternating current net work supply, indicated at 17. The circuit according to this figure comprises three amplifying valves 1, 2 and 8 connected in cascade, in a manner similar to Figure 3. The filaments of the valves 9, 10 and 11 are supplied'each from the secondary windings 14, 13 and 12 respectively of an input transformer 15, these secondary windings being tapped at their mid-points in accordance with the well known practice, the'mid-tap points serving as cathode terminals for the valves and, accordingly, being connected to suitable tap points on the potentiometer 3, in accordance with the spirit of Figure 3. The plate potential voltage is supplied by the further secondary 18 of the input transformer 15, in combination with a rectifier 20 and a smoothing arrangement, the latter being comprised in the usual manner by shunt capacities 22 and 23 and a series inductance 21.

Referring to Figures '7 and 8, I have illustrated an alternative method of practicing the invention by using a push-pull arrangement for the output valves, as 'is most clearly seen from Figure 8, which distinguishes from Figure 4 in that two valves 2 and 8, arranged in push-pulLfashion,

are connected between the high potential diagonal terminals of the bridge system. Each of the grids of the two push-pull valves is connected to one of the remaining diagonal bridge points I and e and the input valve 1 is again arranged in shunt relation to one of the bridge arms, to produce the unbalance and, accordingly, the variation of the plate current of the push-pull output valves. The output translating device, such as loud speaker, telephone, etc., may be either connected to the ends of a resistance 24 adjoining the anodes of the push-pull valves and being tapped at its mid-point for the positive potential supply, as is shown in Figure 8. In this case, it is necessary that the resistance of the translating device, such as loud speaker, is properly designed and adapted for the resistor 24, or vice versa. As an alternative, an audio frequency push-pull transformer 19, of known design, may be used in the plate circuit of the push-pull valves, as shown in Figure '7 and the output translating device connected to the secondary terminals 6 of the transformer, in a wellknown manner. The connection of the valves to the potentiometers 3 and Us seen from Figure 7 and is made in a similar manner to Figure 3 and in accordance with the diagram of Figure 8.

It is also possible to dispense with the part 9 f of the potentiometer '7 according to Figure! and it is furthermore possible, as will be obvious, to combine a push-pull valve circuit with the known circuit according to Figures 1 and 2, in which case the advantage is also obtained of no reaction taking place on the input valve, depending on current changes of the succeeding valve or valves.

It is obvious that many other modifications of my invention are possible, which come under its broader spirit and scope, comprising a bridge system arrangement substantially comprised by resistor elements with the input valve associated therewith, in such a manner as to bring about variable unbalancing of the'bridge system, in accordance with the input current variations.

What I claim is:

1. In a cascade amplification system, comprising a first valve, a second valve to be controlled by said first valve, a direct conductive coupling connection from the output electrode of said first valve to the input electrode of said second valve, a Wheatstone bridge arrangement consisting of two pairs of balanced resistance arms, said first valve being connected to produce an unbalancing of said bridge system in accordance with input current variations, a common high potential operating source for said valves applied to a pair of diagonal points of said bridge system, the remaining pair of diagonal points of said bridge being connected to the input electrodes of said second valve.

2. In an amplification system, a first valve, a second valve to be controlled by said first valve, a

direct conductive coupling connection from the output electrode of said first valve to the input electrode of said second valve, a Wheatstone bridge arrangement comprising two pairs of balanced resistance arms, a high potential source connected to a pair of diagonal points of said bridge for supplying plate current for both of said valves, said first valve being furthermore associatedwith one of said bridge arms to produce an unbalancing of said bridge and said second valve having its input controlled in accordance with potential variations occurring between the remaining pair of diagonal points of said bridge, said system being normally unbalanced for producing a proper operating potential bias for said second valve and means for producing an additional un balancing of said bridge system by said first valve, in accordance with input current variations.

3. In a cascade amplification system, comprising a first valve, a second valve to be controlled by said first valve, a direct conductive coupling connection from the output electrode of said first valve to the input electrode of said second valve, a Wheatstone bridge arrangement com prised of two pairs of balanced resistances, a high potential plate current source for both of said valves connected to one pair of diagonal terminals of said bridge system, the grid bias potential of the second valve being determined by the potential difference existing between the remaining pair of diagonal points of said bridge system and means for producing an unbalancing the output electrode of said first valve to the input electrode of saidsecond valve, a Wheatstone bridge arrangement comprised of two pairs of balanced resistances, a high potential plate current source for both of said valves being connected to one pair of diagonal points of said bridge system, the grid potential of said second valve being determined by the potential difference existing between the remaining pair of diagonal terminals of said bridge system, said first valve being connected in shunt relationship to one of the resistance arms of said bridge to produce an unbalancing of same', in accordance with input current variations applied to the grid of said first valve.

5. In an amplification system, comprising a first valve, a second valve, each having cathode, grid and plate electrodes, a Wheatstone bridge arrangement'comprised. of two pairs of balanced resistances, a high potential plate current source for both of said valves applied to one pair of diagonal terminals of said bridge, a direct conductive coupling connection from the anode of said first valve to the grid of said second valve, further connections whereby the grid potential'of said second valve is determined by the potential difference existing between the remaining pair of diagonal terminals of said bridge system and means for producing an unbalancing of said bridge system by said first valve, in accordance with input current variations applied tothe grid of said first valve.

6. In an amplification system, comprising a first valve, a second valve to be controlled by said first valve, both of said valves including cathode,

grid and plate electrodes, a Wheatstone bridge arrangement comprisedof two pairs of balanced resistors, a plate current high potential source for both of said valves being applied to one pair of diagonal terminals of said bridge, a direct conductive coupling connection from the plate electrode of said first valve to the grid electrode of said second valve, connections whereby the grid bias by said first valve, a plate potential source for both of said valves, at least two potentiometers connected across said plate potential source, connections from the plate electrodes of said valves to tap points on one of said potentiometers, further connections from the cathode electrodes of said valves to tap points on said other potentiometer and a direct conductive coupling connection from the plate electrode of said first valve to the gridelectrode of said second valve.

8. In a cascade amplification system, comprising a first valve, a second valve to be controlled by said first valve, said valves having cathode, grid and plate electrodes, two potentiometers supplying a, gradually varying operating potential, a direct conductive coupling connection from the plate electrode of said first valve to the grid electrode of said second valve, connections from the plate electrodes of said valves to points on fone of said potentiometers and further connections from the cathode electrodes of said valves to points on the other of said potentiometers.

9. In a cascade amplification system, comprising a first valve, a pair of output valves to be controlled by said first valve and connected in push-pull fashion, a balanced bridge system cons'isting of two pairs of balanced resistance arms, a common high potential operating source for said valves applied to a pair of diagonal points of said bridge, means for unbalancing said bridge in accordance with input current variations applied to said first valve, and connections from the control electrodes of said output valves to one of the remaining diagonal points each of said bridge system.

10. In a cascade amplification system, comprising a first valve, at least one pair of second valves connected in push-pull fashion and to be controlled by said first valve, a balanced bridge circuit comprised by two pairs of ohmic resistors with said first'valve associated therewith to produce unbalancing of said bridge in accordance 100 with input current variations applied to said first valve, a high potential operating source for said valves being applied to one pair of diagonal terminals of said bridge system and connections from the control electrodes-of said push-pull valves each to one of the remaining diagonal points of said bridge system. v

11. In a cascade amplification system, comprising a first valve, a pair of second valves connected in push-pull fashion and to be controlled 110 by said first valve, said valves having cathode, grid and plate electrodes, a Wheatstone bridge circuit comprised of two pairs of balanced resistances, said first valve being connected in parallel to one of said resistances for producing an unbalancing of said bridge system in accordance with input current variations" applied to said first valve, a common plate potential source for said valves applied to one pair of diagonal terminals of said bridge system, means including 0 circuit connections to control the grid electrodes of said second valves in opposite phase relation in accordance with relative potential variations occurring between the remaining pair of diagonal terminals of said bridge system.

12. In a cascade amplification system, comprising a first valve, a pair of second valves connected in push-pull fashion and to be controlled by said first valve, a highpotential operating source for said valves, at least one pair of potentiometers associated with said source for providing gradually varying potential drop, a direct conductive coupling connection from the'control electrode of one of said push-pull valves to the plate electrode of said first valve, a further couplingconnection from the control electrode of the remaining of said second valves to a tap point on one of said potentiometers, a further connection from the cathode electrodes of said valves to a further tap point on said last mentioned potentiometer and connections from the anode of said first valve and from the common anode terminal of said push-pull valves to points on the other of said; potentiometers. 

